A portion (Appendices A-1, 2, 3, and 4) of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
This invention relates to a controller integrated with apparatus for the purpose of controlling the operational parameters of the apparatus, and more particularly, to control circuitry operated by processors for a closed loop, self-remedial system. A particular application of the present invention relates to molds utilized with a plastic mold apparatus, such as a hot runner injection mold apparatus.
In the injection mold industry, it is known to have an injection mold apparatus having from one to two hundred plus molding stations. Molds having ninety-six stations are common. Each station is equipped with the functional elements necessary for carrying out the molding process, including a mold cavity, resin feed equipment, and heaters and chilling fluid channels for maintaining resin and the mold in a proper molding condition. In a typical injection mold process, the following steps sequentially take place: (1) the mold cavity closes; (2) molten resin is injected into the closed mold; (3) once the injected resin hardens, the mold is opened; and (4) a formed part is ejected from the opened mold. Each mold cavity can undergo changes in spacing as a result of repeated opening and closing as each finished molded component is ejected. It is critical that the mold return to its precise spacing after the components are ejected and the mold is readied for the next batch. Moreover, in molding components, it is critical to maintain the temperature of the resin within a few degrees of its desired melt temperature in order to achieve the proper quality of the molded components. Given the high production rate of injection molding apparatus, almost instantaneous recognition that the system is out of temperature control is needed.
Conditions in the mold that are important in terms of product quality are the resin temperature, the mold temperature, and mold pressure. If these conditions are out of set ranges, the quality of the final molded components can be adversely affected. For example the molded components can emerge with extraneous plastic along the edges, which is known as xe2x80x9cflashxe2x80x9d.
Other problems incurred with molding parts are variation and inconsistency in the weight and quality of the components.
Conventional techniques for controlling operational parameters of a mold to maintain the parameters within a range of acceptable reference parameters involves monitoring the various parameters with sensors. The data detected by the sensors is transmitted as analog signals to a controller. Certain examples of the prior art are U.S. Pat. No. 5,551,857, issued Sep. 3, 1996, to Osami Fujioka, which discloses controls for a molding apparatus 39 (FIG. 1); and U.S. Pat. No. 5,795,511, issued Aug. 18, 1998, to Peter G. Kalantzis, which discloses a memory function in FIG. 2 for the operational parameters of the hot side 24 of all injection molding machine.
Some of the disadvantages of prior art techniques for monitoring and controlling the operational parameters of a mold apparatus are attributable to the use of large, stand alone controllers, external to the mold. These controllers are expensive, and require large kilowatt power sources and large heavy cables for connection. The cables that provide the kilowatt power generates resistance in the cables and produces unwanted noise that can result in inaccurate signals from the various sensors. Moreover, a large number of connectors need to be made to connect the mold controller to (i) the mold, (ii) the injection mold apparatus, and (iii) a power source. Effecting these connections delays start-up of the equipment, and can contribute to high labor costs for production of molded parts.
In view of the disadvantages of prior art techniques, there is a need for control apparatus, such as for an injection mold apparatus, and methods for operating the apparatus, that permit accurate, automatic and inexpensive control of operational parameters while minimizing production of defective parts.
The present invention is directed to an apparatus and method that satisfy this need. In particular, in one aspect of the present invention, an apparatus has multiple zones, each zone having at least one heater and/or chiller system, and at least one temperature sensor outputting a temperature indicating signal. A power source provides power to any heater, chilling fluid is provided to any chiller system, and a controller controls the temperature of at least some of the zones. The controller comprises a data-receiving processor and a separate control processor. The data-receiving processor receives a temperature indicating signal from each sensor. The separate control processor receives data from the data-receiving processor and controls power provided to the heaters and/or the temperature of chilling fluid provided to the chilling system in response to the data received from the data-receiving processor. Each processor has its own central processing unit.
Typically the apparatus is a hot runner injection mold having multiple injection zones. Typically the controller comprises a closed loop feed back circuit. Preferably the data-receiving processors also calculate the root mean square current of each heater and/or root mean square of the output temperature of fluid from the chiller.
To avoid the problem of the expensive and error-inducing cables, preferably the controller is in a housing, where the housing is mounted on the mold. If necessary, there can be an insulating air gap between the housing and the mold to avoid heat from the mold overheating the processors.
In order to avoid production of defective parts, preferably the apparatus comprises an alarm responsive to an out of control condition such as incomplete ejection of a molded component from the mold, out of specification component quality, irregular spacing between mold components, and incorrect weight of molded components. The apparatus can comprise an automatic or manual control switch responsive to the alarm for shutting down any zone responsible for the out of control condition.
The apparatus typically utilizes a power source providing AC current to the heaters. The control processor compares the actual temperature of each zone against its target temperature. The control processor provides an output signal for regulating the power source, and controls the total number of complete current cycles provided by the power source to each heater. This is accomplished by calculating the number of current cycles required to achieve the target temperature based on the difference between actual temperature and target temperature, and then comparing the number of current cycles needed against the actual number of cycles being provided to each heater. For this purpose, the apparatus includes a detector, such as a transformer, for detecting the amount of current provided to each heater.
The data-receiving processor and control processor preferably are on separate printed circuit boards. Because the data-receiving function of data received from the heaters and the current sensors is processor-intensive for a large number of zones, the data-receiving processor can comprise separate processor modules, each module having its own CPU. For example, for 96 zones each module can be used for 48 zones.
In a preferred control processor, the amount of power needed for each heater is determined with a PID calculation, where a range of limits is provided for the amount the power to be applied, independent of the PID calculation to prevent over heating the system.
To minimize the calculation load on the first processor, preferably the RMS value for detected current utilizes an algorithm to calculate the sum of squares which requires less processor capacity, as detailed below.
To be certain, for power efficiency, that AC power is provided in complete cycles, the system includes a zero cross-over detector and controls for starting and ending power application at zero cross-over.
Optionally, the present invention monitors each molding station with a sensor during the resin injection step. If the sensor detects that the mold is improperly opening during the injection step, which can cause flash, the system automatically increases the pressure utilized to keep the mold closed.
The present invention also includes a system and method for starting up the apparatus, so that all zones reach a startup target temperature at substantially the same time. This is effected by determining a heat-up time for each zone, where the heat-up time is equal to the amount of time required to heat up the adjacent portion of the apparatus to its start-up temperature with the heater on substantially full power. The heat-up times of the heaters are compared to identify the longest heat-up time. The amount of heat-up power for each heater is determined, the heat-up power for each zone being the amount required to heat up the respective adjacent portions of the apparatus to its start-up temperature in the heat-up time. Then there is applied to each heater simultaneously its respective heat-up power, with the result that all portions of the apparatus reach their target temperature at substantially the same time.
In a more general sense, a system according to the present invention controls the operation of an apparatus having a plurality of operating zones, where the system comprises at each zone, at least one operating element (such as a heater), and a detector (such as a transformer) generating an operating signal representative of the operating state of the operating element (such as power to the heater). An input is provided for each operating element to vary an operational parameter (such as temperature) of each zone. A sensor senses the operational parameter of each zone and generates a corresponding analog sensing signal. The system includes a controller that comprises a first processor for digitizing the signals and determining from the signal the operating state of the operating element and the operational parameter of each zone. The controller also includes a second processor for performing a PID calculation for each zone based on the operating state and the operational parameter to determine the amount of input of each operating element to maintain the operational parameter of each zone within a selected range. The second processor preferably limits the maximum amount of input determined by the PID calculation to prevent damage to the apparatus and operating elements.
As a result of the combination of these features of the present invention, including separate processors for data-receiving and control, a controller mounted directly on the apparatus without cables, an efficient algorithm for calculating RMS values, use of full AC cycles for power to the heaters, and a system for detecting out of control conditions, it is possible to efficiently, effectively, and automatically manufacture molded parts.